Bioelectrochemical treatment of gaseous byproducts

ABSTRACT

The present invention relates to a method for producing electrical energy or hydrogen gas from a gas stream containing one or more gaseous compounds that are oxidatively degradable by microbes, the method comprising contacting the gas stream with an anode of a bioelectrochemical device, said anode containing said microbes which oxidatively degrade one or more of said gaseous compounds while producing electrical energy or hydrogen gas by said oxidative degradation, wherein: (i) said anode is sufficiently porous such that gas is permitted to flow therethrough, (ii) said anode contains on its surface and/or interior portions a proton-conducting medium, and (iii) said anode is in electrical communication with a cathode of the bioelectrochemical device. The invention is also directed to a bioelectrochemical device (e.g., microbial fuel cell) configured to accomplish the above method.

This invention was made with government support under Contract NumberDE-AC05-000R22725 between the United States Department of Energy andUT-Battelle, LLC. The U.S. government has certain rights in thisinvention.

FIELD OF THE INVENTION

The present invention relates to the field of bioelectrochemicaltreatment of gaseous products. More particularly, the present inventionrelates to bioelectrochemical (e.g., microbial fuel cell) treatment ofsulfide-containing or carbon monoxide-containing gas streams, such asthose occurring from a petroleum refining process or syngas-producingprocess.

BACKGROUND OF THE INVENTION

Gaseous streams emanating from industrial processes often containnumerous gaseous byproducts that are processed as waste or directlyreleased into the environment. The gaseous waste is often a significantfinancial liability since the waste is typically required to beprocessed by some measure before being released or disposed of. Thegaseous waste also presents an environmental liability since either rawor processed gaseous waste ultimately enters the environment. For thisreason, there would be a significant advantage if such gaseous wastecould be utilized by a process which uses such gaseous waste to produceenergy. There is, therefore, an ongoing effort to find new ways ofefficiently utilizing gaseous waste products in a process that producesa net energy output.

For example, sulfurous compounds and materials are commonly found inunrefined petroleum sources. These compounds are undesirable inpetroleum products for a number of reasons, including their readyconversion to sulfur dioxide (SO₂) gas when included in fossil fuelsundergoing combustion (as in an automobile or power plant), as well astheir ability to function as poisons to noble metal catalysts used incatalytic reforming units and catalytic converters. Accordingly, thesesulfurous compounds are desirably removed during the petroleum refiningprocess.

The most widely used process for removing sulfurous compounds frompetroleum products is the hydrodesulfurization (HDS) process. In the HDSprocess, petroleum intermediates are subjected to hydrogenationconditions, typically by contacting the petroleum intermediates with aspecialized catalyst (typically, an alumina carrier impregnated withcobalt-modified molybdenum sulfide) in a fixed-bed reactor at elevatedtemperatures (e.g., 300 to 400° C.) and elevated pressures (e.g., 30 to130 atmospheres). The predominant sulfurous product resulting from theHDS process is hydrogen sulfide (H₂S), a toxic and highly malodorousgas, the bulk of which is desirably converted to other sulfur-containingcompounds that are either more commercially useful or more convenient tohandle and transport.

The hydrogen sulfide produced by the HDS process is most commonlyprocessed by the Claus process. In the Claus process, H₂S is convertedto elemental sulfur in a multi-step process. One step is a thermal step(conducted typically at above 850° C.) in which a portion (approximatelyone-third) of the H₂S is converted to SO₂ by the following reaction:

2H₂S+3O₂→2SO₂+2H₂O

Another step in the Claus process is the reaction of H₂S and SO₂ to formelemental sulfur.

2H₂S+SO₂→3S+2H₂O

The above reaction is typically conducted at elevated temperatures inseveral stages (e.g., 315-330° C. in a first stage, 240° C. in a secondstage, and 200° C. in a third stage) in the presence of a catalyst(e.g., activated alumina or titanium dioxide).

An alternative process for removing sulfurous compounds from petroleumproducts is the Merox process. In the Merox process, petroleum feedstockis treated with caustic solution (typically NaOH solution) which reactswith thiols (represented herein as RSH where R is an organic group)therein according to the following reaction:

2RSH+2NaOH→2NaSR+2H₂O

If only extraction of the sulfurous compound is desired, the process canbe halted at this stage. If precipitation of solid sulfurous compoundsis desired along with regeneration of the caustic, a second step isemployed according to the following reaction to produce insolubledisulfide compounds:

4NaSR+2O₂+2H₂O→2RSSR+4NaOH

The Merox process also generally requires a separate pre-treatment(pre-washing) step for the removal of any hydrogen sulfide that may bepresent in the petroleum feedstock because H₂S will poison thecirculating caustic solution used for reaction with thiols. Typically,this is accomplished by pre-treating the feedstock with a causticaccording to the following reaction:

H₂S+NaOH→NaSH+H₂O

The HDS process, the Claus process, and Merox process (and otherprocesses) are energy demanding and rely on fossil fuels. Usually,extraordinary amounts of water are used in the process, and a largeamount of this water is wasted or is contaminated and must be treated aswaste. Means for recovering or cleansing water add to the energy demand.For example, the Claus process is known to generate over 2.6 tons ofsteam for each ton of sulfur produced; and in 2005, approximately64,000,000 metric tons of sulfur was produced from petroleum refineries.

Carbon monoxide-containing gas streams can either be an industrial wastestream or produced specifically for generation of energy. Some examplesof carbon monoxide-containing waste gas streams include those emanatingfrom coal processing or coal burning plants, and other combustionprocesses. Processes designed for generation of energy via gasificationof coal and/or biomass produce a mixture of hydrogen and carbon dioxidedescribed as synthesis gas (syngas) or producer gas, depending on thehydrogen content. The syngas may also be produced by steam-methanereforming. Typically, a complex and energy intensive process, such as acatalytic water-gas shift (WGS) reaction, is used to convert the carbonmonoxide to carbon dioxide and hydrogen according to the generalreaction scheme:

CO+H₂O→CO₂+H₂

In a typical WGS process, carbon monoxide (CO) and water (i.e., steam)are reacted in at least two major steps: the high temperature shift(HTS) step conducted at a temperature of about 350-370° C. in thepresence of a metallic catalyst (typically, iron oxide promoted withchromium oxide), followed by a low temperature shift (LTS) stepconducted at a temperature of about 190-220° C. in the presence of ametallic catalyst (e.g., copper on a mixed support of zinc oxide andaluminum oxide). The LTS step functions to achieve higher conversions ofCO to H₂ than that attainable in the HTS step.

Accordingly, there would be a benefit in a method generally useful inderiving a net surplus of energy from common gaseous waste products,such as mercaptans and carbon monoxide. The process preferably convertsthese gaseous byproducts to harmless products by a process that is lessenergy demanding and more cost efficient. There would be an addedbenefit to such a process which also produces significantly less waste(including aqueous waste) and utilizes less water. There would be anadditional benefit in such a process which also does not rely on fossilfuels, i.e., which operates by renewable means.

SUMMARY OF THE INVENTION

The present invention is directed to a novel method for utilizinggaseous compounds that are oxidatively degradable by microbes (forexample, gaseous mercaptan compounds and/or carbon monoxide, i.e., CO)from a gas stream containing such compounds for the purpose of producingenergy therefrom (i.e., electricity or hydrogen gas), and in theprocess, reducing the levels of these byproducts. The method involvestreating the gas stream with a bioelectrochemical system or device(i.e., BES) by contact of the gas stream with an anode therein whichcontains microbes capable of oxidatively degrading one or more gascompounds, preferably mercaptan compounds and/or CO, while producingelectrical energy or hydrogen therefrom. The anode is suitably porous topermit flow of the gas stream therethrough such that the gas streaminterfaces with the surface of the anode. The anode also containstherein a proton-conducting medium such that protons being produced bythe oxidative degradation of the gaseous compounds can migrate from theanode to the cathode. The proton-conducting medium also functions tosolubilize the gaseous compounds, such as H₂S, other mercaptans or CO,in the gas stream for more efficient processing by microbes on the anodesurface. In the use of the device for electricity production, the anodeis in electrical communication with a cathode of the device. Acation-permeable material is preferably in direct contact with the anodeand cathode and separates the anode and cathode such that the gas streamcontacting the anode does not contact the cathode.

The produced electrical energy can be used to power any suitableprocess, and more preferably, a process from which a mercaptan-laden orCO-laden gas stream emanates, e.g., a fuel refining, biomass-processing,or steam-methane reforming process. When applied to a mercaptan-ladengas stream, the method is particularly directed to the case where themercaptan compound is hydrogen sulfide (H₂S).

The invention is also directed to a device (or system) containing one ormore BESs for accomplishing the inventive method. The invention is alsodirected to methods of operating the BES for optimal performance in sucha process.

In another embodiment, the BES device is operated in such a manner thathydrogen is produced at the cathode during microbial consumption of oneor more mercaptan compounds or CO at the anode. Such a device is alsoreferred to herein as a microbial electrolysis cell (MEC). The hydrogengas may be utilized as a chemical commodity or processing reactant, ordirectly as a fuel for the production of electricity to power one ormore processes, preferably one or more processes from which themercaptan- or CO-laden gas stream emanates.

In still another embodiment, the BES device is operated in such a mannerthat the cathode electrochemically reduces one or more electrochemicallyreducible species, such as a nitrate, chlorate, or reducible metalspecies, during microbial consumption of one or more mercaptan compoundsor CO at the anode.

Thus, as will be described in further detail below, the methodadvantageously provides a method for removing mercaptans (particularly,hydrogen sulfide) and/or CO in a gas stream by a renewable process whichis significantly less energy demanding and more cost efficient thancurrent technologies. The method also advantageously converts mercaptansand CO to environmentally benign and/or useful products while producingsignificantly less waste (or no waste) and using minimal amounts ofwater.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Depiction of a horizontal gas-phase BES configuration with useof a cation-exchange membrane for separating anode from cathode.

FIG. 2. Depiction of a vertical gas-phase BES configuration with use ofa filter or membrane for separating anode from cathode.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the invention is directed to a method for removingoxidatively degradable gaseous compounds, such as mercaptan (i.e.,thiol) compounds and/or CO, from a gas stream by treatment of the gasstream with one or more BESs. The BES considered herein is any reactorsystem in which electrons are transferred by microbes to a conductivesurface (i.e., by an exoelectrogenic mechanism) during the course ofmicrobial consumption of gaseous byproducts. The transferred electronsare directed to the production of energy, e.g., either electricity orhydrogen production. In a particular embodiment, the BES is a microbialfuel cell (MFC) or microbial electrolytic cell (MEC).

The method preferably involves flowing a mercaptan- or CO-laden gasstream through a porous anode of the BES, wherein the anode containsmicrobes residing thereon capable of utilizing the mercaptan compoundsor CO as a nutritive and energy source. Catabolism of mercaptancompounds by the microbes results in their oxidative degradation into,typically, non-gaseous (solid or liquid) compounds that are less toxicand more easily storable and transportable. Catabolism of CO by themicrobes at the anode occurs according to the partial reaction:

CO+H₂O→CO₂+2H⁺+2e⁻

wherein either electricity is produced with no release of H₂ gas (i.e.,under aerobic conditions) or H₂ gas is produced at the cathode insteadof electricity (i.e., under anaerobic conditions). Furthermore, asdescribed in further detail below, one or more reducible species may bereduced by the produced electrons instead of or in addition toproduction of electricity or H₂ gas.

The gaseous mercaptan compounds are any compounds that contain one ormore thiol (—SH) groups. Typically, the most predominant and commongaseous mercaptan compound in a gas stream is hydrogen sulfide (H₂S). Inone embodiment, the mercaptan compound is essentially only H₂S (e.g., atleast 99% by volume with respect to other mercaptan compounds). Inanother embodiment, the mercaptan compounds include predominantly H₂S incombination with organomercaptan compounds. Some examples oforganomercaptan compounds include methanethiol, ethanethiol,1-propanethiol, 2-propanethiol, ethanedithiol, thiophenol, and the like.In a less common embodiment, the mercaptan compounds include one or moreorganomercaptan compounds in a greater amount than H₂S.

A gas stream containing one or more mercaptan compounds can be any gasstream requiring removal of mercaptans. Particularly considered hereinis a gas stream emanating from a petroleum refining process. Thepetroleum refining process can be, for example, a hydrodesulfurization,pyrolysis (i.e., fluid catalytic cracking), Claus, or Merox process. Thepetroleum source being refined can be, for example, crude fossil fuel orpetroleum (i.e., feedstock), petroleum coke, crude oil, natural gas,liquified natural gas (LPG), naphtha, kerosene, jet fuel, bitumen,gasoline, fuel oil, diesel fuel, coal, or a derivative or modified formthereof. The mercaptan-laden gas stream can reach the BES directly fromthe mercaptan-evolving process, or alternatively, after any number ofother process steps. Other process steps may include, for example,heating, cooling, pressurization, depressurization, liquid extraction,gas separation, filtration, adsorption, and the like. A mercaptan-ladengas stream can result from other processes as well, any of which canalso be treated by the BES described herein. Other mercaptan-producingprocesses include, for example, production of chemicals (e.g., plastics,polymers, textiles, pharmaceuticals, fine or bulk chemicals) andcombustion of fossil fuels (e.g., as occurs in power plants andautomobile use).

The microbes of the BES are capable of oxidatively degrading one or moreoxidatively degradable gaseous compounds, such as one or more mercaptancompounds and/or CO. The mercaptan compounds can be partially oxidizedto such species as elemental sulfur or sulfur oxides. More preferably,the mercaptan compounds are completely oxidized to sulfate compounds.These compounds are typically solid in form and soluble in water, andcan thus be readily removed upon their formation from a gas stream. Forexample, H₂S can be oxidatively treated by a BES according to thereaction shown below, wherein the sulfate can be removed via limeprecipitation. In addition, the water used in the process can berecycled by removal of remaining dissolved calcium with acation-exchange resin.

H₂S+4H₂O→10H⁺+SO₄ ²+8e⁻

The gas stream entering the BES can have any particular concentrationlevel or range of concentrations of mercaptan compounds or CO. The totalor individual concentration of one or more mercaptans or CO can be, forexample, at or less than (or at or greater than) 5 ppm, 10 ppm, 20 ppm,30 ppm, 40 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 500 ppm, 750ppm, 1,000 ppm, 1,250 ppm, 1,500 ppm, 2,000 ppm, 2,500 ppm, 3,000 ppm,5,000 ppm, 7,000 ppm, or 10,000 ppm. Any ranges resulting from any ofthe values given above are also considered herein. One or more of theBESs (or MFCs) being used for treating the mercaptans or CO may properlyfunction (or most effectively function) within a select range ofmercaptan or CO concentration, or below a specified concentration, orabove a specified concentration. Accordingly, the invention alsocontemplates using more than one BES (or MFC) in a process, wherein oneBES (i.e., a first BES) may be used to treat a gas stream containing ahigh concentration of mercaptans or CO (e.g., at or greater than 1,000ppm) while another BES (i.e., a second BES) is used to treat a gasstream containing a low concentration of mercaptans or CO (e.g., at orless than 1,000 ppm). The BESs or MFCs may be interconnected such thatthe low-mercaptan or low-CO gas stream being fed into the second BES orMFC is processed gas exiting the first BES or MFC.

By “removal” or “cleansing” of the gas stream of mercaptan compounds orCO is meant that the concentration (i.e., level) of one or moremercaptan compounds or CO is appropriately reduced to meet the needs orrequirements of a particular application. For most applications, the gasstream is cleansed such that the gas stream experiences at least a 20%reduction in one or more mercaptan compounds or CO. More preferably, thegas stream is cleansed such that the gas stream experiences at least a30%, or 40%, or 50%, or 60%, or 70%, or 80%, or 90%, or 95% reduction inone or more mercaptan compounds or CO. Even more preferably, the gasstream is cleansed such that one or more of the mercaptan compounds orCO have been substantially removed from the gas stream, i.e., greaterthan 95% reduction in concentration (more preferably, at least 98%, 99%,99.5%, or 99.9% reduction in concentration). For example, in differentembodiments, the cleansed gas stream may contain a mercaptan or COconcentration up to or less than 200 ppm, 150 ppm, 100 ppm, 50 ppm, 30ppm, 20 ppm, 15 ppm, 10 ppm, 5 ppm, 1 ppm, 0.5 ppm, 0.1 ppm, or 0.05ppm.

As used herein, and as generally understood in the art, “microbial fuelcells” (i.e., MFCs) are fuel cells which operate by using microbes(i.e., microorganisms) that possess the ability to donate electrons tothe anode of the fuel cell during microbial oxidative degradation ofcompounds in order to produce electricity. Such microorganisms are knownas exoelectrogenic organisms. Exoelectrogenic organisms can donateelectrons to the anode in either of two ways: via mediators (e.g., thenumerous dyes used in the art for this purpose) or in the absence ofmediators (i.e., a mediator-less MFC).

An MFC contains an anode and a cathode that are in electricalcommunication, typically by interconnection with an electrical conductor(e.g., by a metal wire). The microorganisms in contact with the anodeoxidatively catabolize the mercaptan compounds or CO to produceelectrons and protons (H⁺ ions), as well as oxidized species, e.g.,oxidized mercaptan compounds or CO₂, and possibly, oxidized hydrocarbonmaterial. The electrons are attracted to the anode and travel to thecathode. At the same time, the produced protons migrate toward thecathode. At the cathode, oxygen gas (typically from air) reacts with theelectrons and protons to produce water according to the reaction:

O₂+4H⁺+4e⁻→2H₂O

According to the present invention, a gas stream (as opposed to aliquid) is being processed by the MFC. In order to effectively andefficiently process a gas stream by direct interface with the anode, theanode is sufficiently porous such that flow of the gas stream ispermitted through the anode. The porosity of the anode permits gas flowtherethrough such that gaseous components (e.g., mercaptan and/or COcomponents) of the gas stream intimately contact surface portions of theanode and allow for efficient interaction of the microbes residingthereon with the gaseous components. The gas stream can also be made torecirculate through or around the anode by convection.

The porosity of the anode is such that the gas stream can flow throughthe anode at a viable gas flow rate. Preferably, the porosity of theanode permits a gas flow rate of at least 0.5 mL/min, 1.0 mL/min, 2.0mL/min, 3.0 mL/min, 4.0 mL/min, 5.0 mL/min, 6.0 mL/min, 7.0 mL/min, 8.0mL/min, 9.0 mL/min, or 10 mL/min. In particular embodiments, theporosity of the anode can permit higher flow rates, e.g., at or above 15mL/min, 20 mL/min, 25 mL/min, 30 mL/min, 35 mL/min, 40 mL/min, 45mL/min, 50 mL/min, 100 mL/min, 200 mL/min, 300 mL/min, 400 mL/min, or500 mL/min. The anode preferably has a porosity value of at least about0.3 (and more preferably at least about 0.4, 0.5, 0.6, 0.7, 0.8, or0.9), wherein the porosity value recited herein is calculated as thevolume of void space over the total (i.e., bulk) volume. The foregoingporosity values can be recited as percentages (e.g., 0.3 corresponds to30%).

The sizes of the pores present in the anode can be any suitable poresize or pore size range. The pore sizes can be, for example, at or above(or at or below) 20 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm,750 nm, 1 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm, 50μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 200 μm, 300 μm, 400 μm, or 500μm. Any ranges resulting from any of the above exemplary values are alsoconsidered herein. The pore size distribution can be essentiallymonodisperse (e.g., a 10% or less deviation from a particular pore sizevalue), or alternatively, polydisperse. The anode can also possess adual pore size distribution. For example, the anode may possess a dualpore size characterized by the presence of one distribution of pores ofa first size or size range and a second distribution of pores of asecond (different) size or size range. The pore size distribution canalso be characterized by more than two (i.e., multiple) different sizesor size ranges. Dual or multiple pore size distributions can beappropriately selected to control or modify the flow of the gas streamthrough the anode.

Other aspects of the anode that can be modified, optimized, orcontrolled include the gas flow rate/retention time, gas recirculation(i.e., preferably for complete consumption of gaseous reactant), thegas:liquid flow rate ratio (i.e., to control liquid level), liquidrecirculation, and pressure in the anode chamber. The anode ispreferably under a slight positive pressure to facilitate liquid flow.The anode is also preferably designed to include two electrodematerials, one in the bulk anode chamber and the other lining themembrane. The ratio of these two chambers is typically ˜10:1.Preferably, two zones are provided, a gas-continuous zone and aliquid-continuous zone. This is preferably done by controlling placementof gas spargers and porosity of the anode electrode material alongsidethe membrane. A low porosity hydrophilic material promotes formation ofa liquid-continuous zone given that the liquid flow rate is sufficientlyhigh. The liquid is then recirculated into the gas-continuous zone. Bythis preferred method, proton transport and gas-liquid contact (ingas-continuous zone) can be significantly improved.

The gas outlet preferably includes a pressure control valve which opensonly if gas pressure rises above a certain threshold, e.g., 0.1 atm.This would enable a positive pressure on the liquid, therebyfacilitating its flow out of the chamber via the electrode zonealongside the membrane, thus ensuring efficient gas utilization. The gasis preferably distributed via a distributor throughout the anodechamber. Similarly, the liquid is preferably introduced via adistributor to provide a trickle bed operation that allows for maximumgas-liquid contact.

The surface area to volume ratio (specific surface area) for the anodeis preferably at least 1,000 m²/m³. As understood in the art, the anodevolume being considered in calculating specific surface area does notinclude void space of the anode, but rather, only the volume that themass of the anode material itself occupies. The specific surface area ismore preferably at least 5,000 m²/m³, more preferably at least 10,000m²/m³, more preferably at least 30,000 m²/m³, more preferably at least40,000 m²/m³, and even more preferably at least 50,000 m²/m³. Inparticular embodiments, the specific surface area is preferably at leastabout 100,000 m²/m³, 200,000 m²/m³, 300,000 m²/m³, 400,000 m²/m³,500,000 m²/m³, 600,000 m²/m³, 700,000 m²/m³, 800,000 m²/m³, 900,000m²/m³, 1.0×10⁶ m²/m³, 2.0×10⁶ m²/m³, 3.0×10⁶ m²/m³, 4.0×10⁶ m²/m³, or5.0×10⁶/m³. Any ranges of specific surface areas that can result fromany of the values set forth above are also contemplated herein.

The anode can be constructed of any electrically conductive materialknown in the art suitable for the purposes described herein. The anodematerial is preferably amenable to the growth and adherence ofgas-processing microbes. Some classes of electrode materials (or acoating thereof) include conductive metals (e.g., nickel, palladium,platinum, silver, gold, titanium, cobalt, tungsten, stainless steel, andalloys thereof), conductive polymers, or a metal-deposited carbon anode(e.g., Pt-deposited carbon anode). Particularly preferred for the anodeare electrodes based on conductive carbon. Typically, any elemental formof carbon is suitable as a conductive carbon material. Some examples ofcarbon electrodes include carbon fiber, carbon paper, carbon foam (e.g.,reticulated vitreous carbon), carbon cloth, carbon felt, carbon wool,carbon granules (e.g., granular activated carbon), carbon brushes,graphite, or a combination thereof. In a particular embodiment, theanode is in the form of a packed bed of any of the carbon formsdescribed above, e.g., a packed bed of granular graphite. The conductivecarbon material can have any additional suitable physicalcharacteristics, such as having a powderized, grainy, fibrous,nanotextured, or patterned texture. The conductive carbon material canalso be of a less typical form of carbon, such as carbon nanotubes(e.g., single or double walled) or fullerenes. The anode can also haveany of the three-dimensional architectures known in the art that areknown to possess high porosity values and high flow-through rates.Alternatively, the anode can have a flat (e.g., planar ortwo-dimensional) topology, though this is typically less preferred forthe purposes of the present invention.

The anode can also have any suitable shape. The shape of the anode canbe, for example, generally planar (e.g., 50 cm×50 cm×5 cm),block-shaped, columnar, spherical (e.g., 4 cm to 40 cm diameter), ovoid,or cuboidal (e.g., 1 cm×1 cm×1 cm or 20 cm×20 cm×20 cm). The anode canalso be layered or segregated by containing layers or regions of thesame or different anode materials.

In order to facilitate interaction of the gaseous mercaptan compounds orCO with the anode, the anode contains on its surface a proton-conductingmedium. The proton-conducting medium possesses the property ofconducting protons (or permitting passage or migration of protons) fromthe anode to the cathode. By being “on the surface” of the anode ismeant that the proton-conducting medium is available for intimateinteraction with the gas stream, and thus, can mean that theproton-conducting medium is on outer surface portions of the anode,inner (i.e., interior) surface portions of the anode, selected inner orouter surface portions of the anode, or a combination thereof.

In a preferred embodiment, the proton-conducting medium is aproton-conducting liquid. The proton-conducting liquid can also hereinbe referred to as the “aqueous phase” or “continuous liquid film”. Theproton-conducting liquid can be any polar liquid, e.g., water, alcohol(e.g., methanol, ethanol, or isopropanol), or other polar solvent orcombination of solvents capable of conducting protons. Preferably, theliquid also has an ability to dissolve CO or a gaseous mercaptancompound to be processed (e.g., particularly H₂S, which is particularlysoluble in most aqueous-based liquids and solutions). The aqueous-basedliquid can be used without a dissolved solute, or alternatively, withany suitable solute. The solute can be, for example, a buffer,conductivity enhancer, or pH adjuster. Some examples of solute include,for example, alkali halide salts (e.g., NaCl and KCl), sulfates,ammonium salts, nitrates, phosphates, ethylenediamine tetraacetate(EDTA), tris(hydroxymethyl)-aminomethane (Tris),3-morpholinopropane-1-sulfonic acid (MOPS),4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid (HEPES), sodiumcitrate, and the like. The concentration of the solute can be anysuitable concentration, e.g., 1 M, 0.5 M, 0.4 M, 0.3 M, 0.2 M, 0.1 M, 50mM, 20 mM, 10 mM, or 1 mM; or concentrations above or below any of thesevalues, or any range between these values. An example of a particularlysuitable aqueous solution is a potassium phosphate buffer solution(e.g., 100 mM at a pH of 7.5).

In one embodiment, a mediator, such as ABTS, can also be included in theproton-conducting liquid. The molar concentration of mediator can be anysuitable concentration, e.g., at or above (or at or below) 1 μM, 10 μM,25 μM, 50 μM, 100 μM, 200 μM, 300 μM, 400 μM, 500 μM, 1 mM, 10 mM, 25mM, 50 mM, 100 mM, 200 mM, 300 mM, 400 mM, 500 mM, 1 M, 1.5 M, or 2 M.In another embodiment, a mediator is excluded from the proton-conductingliquid.

In the case of a proton-conducting liquid, it is preferable for theproton-conducting liquid to form a film on the surface of the anode. Thefilm can be located on, for example, the anode's outer surface, innersurface, or a combination thereof, or selected portions thereof.Preferably, the film of liquid has a thickness of at least 0.5 μm (0.5microns). In different embodiments, the liquid film may preferably havea thickness of at least 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm,9 μm, or 10 μm. Preferably, the film of liquid has a thickness of nomore than 20 μm. In different embodiments, the liquid film maypreferably have a thickness of no more than 15 μm, 10 μm, 9 μm, 8 μm, 7μm, 6 μm, 5 μm, 4 μm, 3 μm, 2 μm, or 1 μm. Any range resulting from anycombination of minimum and maximum values given above is also consideredherein.

The anode (and optionally, the cathode) can also be filled to a certainextent with the proton-conducting liquid, as long as less than 100% ofthe pore volume of the anode is occupied by the liquid in order that avoid volume remains for gas circulation. In other words, the occupiedpore volume ratio is less than 0.1 (or less than 100%). In differentpreferred embodiments, the liquid may occupy 95%, 90%, 85%, 80%, 75%,70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5%, 2%,or 1% of the total pore volume, or an amount greater than or less thanany of the foregoing values, or a range between any of the foregoingvalues. When a proton-conducting liquid is used, it typically needs tobe replenished to maintain a coating on the anode surface. Thereplenishment may be conducted in a way that maintains a desired liquidfilm thickness or occupied pore volume ratio.

In another embodiment, the proton-conducting medium is aproton-conducting solid. The proton-conducting solid has the ability toconduct protons while in the solid state. By being in a “solid state”,the material does not flow or appreciably deform under the conditionsused in the process. The solid can also be a semi-solid, amorphoussolid, or a supercooled liquid, such as a glass. The proton-conductingsolid can be, for example, a solid oxide material (e.g.,yttrium-stabilized zirconia (YSZ) or gadolinium-doped ceria (GDC)), astraditionally used in solid oxide fuel cells. The proton-conductingsolid can also be, for example, an ionomer composition, such as thosecontaining perfluorovinyl groups and/or sulfonic acid groups (e.g.,Nafion®). In addition, a solid proton-conducting medium can be used incombination with a liquid proton-conducting medium.

Particularly in the case where the anode is constructed of a hydrophobictype of material (e.g., a carbon form), the anode is preferably renderedsufficiently hydrophilic to permit favorable interaction (i.e.,adherence, interfacing, or bonding) of the polar proton-conductingmedium with the anode material. Any method for coating the surfaces ofthe anode to render them sufficiently hydrophilic is applicable herein.For example, the anode surface can be powder-coated, spray-coated, ordip-coated with one or more hydrophilic polymeric or molecularmaterials, and optionally thermally processed, calcined, or dried. Theanode can also be treated by ionizing radiation or an ion beam processfor this purpose. In a preferred embodiment, the anode is treated by aplasma process to render its surface hydrophilic. The plasma process ispreferably an oxygen plasma process.

The anode may also be configured to reduce or prevent moisture loss. Forexample, the anode may be suitably coated with or encapsulated by acoating that helps to retain moisture. The coating can be any made ofany suitable organic or inorganic material useful for this purpose.

The cathode can be constructed of any suitable electrically conductivematerial, such as any of the materials described above for the anode.The cathode can also have any of the properties (e.g., porosity,specific surface area, and so on) described above for the anode. Thecathode may also include a biological system capable of transferring orutilizing electrons. In one embodiment, the cathode is any of the gascathodes known in the art (e.g., a Pt/air electrode). In order to permitproton transfer from the anode to the cathode, the cathode should alsobe coated with a proton-conducting liquid (or solid). In one case,wetness is maintained on the cathode by encouraging theproton-conducting liquid (wetting agent) to flow from the anode to thecathode (i.e., through a cation-permeable material or membrane), whereinperiodic wetting of the anode functions to also wet the cathode. Inanother case, wetness is maintained on the cathode by periodic additionof a wetting agent directly onto the cathode, either with or without aflow of wetting agent from the anode.

In a MFC of the invention, a cation-permeable material is preferablysituated between the anode and cathode and in direct contact with eachsuch that the anode and cathode are separated to the extent that the gasstream being processed by the anode does not appreciably contact thecathode. More preferably, the cation-permeable material substantiallyeliminates or completely prevents contact of the gas stream with thecathode. The cation-permeable material preferably allowsproton-conducting liquid described above (if a proton-conducting liquidis used) to pass through to the cathode in order that protons areefficiently transported to the cathode. The cation-permeable material istypically in the form of a layer of any suitable thickness, e.g., 0.1μm, 0.5 μm, 1 μm, 10 μm, 100 μm, 1 mm, 10 mm, or 100 mm, or values lessthan or greater than any of the foregoing values. The cation-permeablematerial can be any material which permits proton transport from theanode to the cathode while substantially preventing contact of thecathode with the gas stream. For example, the cation-permeable membranecan be filter paper, cloth, or a specific pore-sized filter material,such as a 0.2 micron filter material.

Preferably, the cation-permeable material is a cation-selectivepermeable material, which is also referred to herein as a cationexchange material. The cation exchange material can also be, morespecifically, a proton exchange material. More preferably, the cationexchange or proton exchange material is in the form of a membrane, i.e.,a cation- or proton-selective permeable membrane or cation or protonexchange membrane (PEM). In particular, a PEM selectively allows thediffusion or passage of hydrogen ions (H⁺, otherwise referred to hereinas “protons”) while not allowing the passage of anions, includingelectrons. The cation-selective permeable material should alsosubstantially prevent oxygen from diffusing from the cathode side intothe anode side. The cation-selective or proton-selective permeablematerial can be any such material known in the art having theseproperties. Any of the PEMs known in the art can be used herein, andmore particularly, those belonging to the class of ionomerpolyelectrolytes having these properties, such as the Nafion® class ofPEMs. The cation-selective material can alternatively be in the form ofa cation- or proton-selective salt bridge, or a glass bridge containinga cation or proton exchange membrane.

The spacing between the anode and cathode (i.e., the electrode spacing)can be any suitable spacing. In one embodiment, the spacing is withinthe range of 0 to 1 cm. Smaller electrode spacings (i.e., less than 1cm) can also be used. For example, in different embodiments, theelectrode spacing can be at about or less than 0.8 cm, 0.5 cm, 0.25 cm,0.1 cm, 5 mm, 4 mm, 3 mm, 2 mm, 1 mm, or 0.5 mm. In another embodiment,the electrode spacing is greater than 1 cm, and can be, for example, ator greater than 2 cm, 5 cm, 10 cm, 20 cm, or 30 cm. The spacing betweenthe electrodes can be at least partly determined by the thickness of acation-permeable material separating the electrodes.

Preferably, in order to maximize electrical output and provide anefficient system for electricity production, the level of oxygen in thereaction zone of the anode is reduced, and preferably substantiallyreduced, so as to result in an appreciably anaerobic environment at theanode. The gas streams being particularly considered herein (e.g.,emanating from fuel processing or refining operations) generally do notcontain oxygen, or at most, trace levels of oxygen. A trace level ofoxygen generally corresponds to no more than about 100 ppm, and morepreferably at or less than, for example, 50 ppm, 40 ppm, 30 ppm, 20 ppm,10 ppm, 5 ppm, 1 ppm, 0.5 ppm, or 0.1 ppm. If more than trace levels ofoxygen are in the gas stream, any method for removal and exclusion ofoxygen at the anode can be used to partially or completely removeoxygen. For example, an inert gas, such as nitrogen, can be added to thegas stream to reduce the level of oxygen to a trace level. Any of theoxygen scavenging processes known in the art can also be used, such asthose based on elemental iron, or enzymes, or catechol, to partially orsubstantially remove oxygen from a gas stream.

The MFC (or BES) described herein can have any suitable number ofcathodes and anodes. For example, the MFC (or BES) can be operated withone anode and more than one cathode, or one cathode and more than oneanode, or an equivalent number of anodes and cathodes (e.g., two anodesand two cathodes, or three anodes and three cathodes). In addition, theMFC (or BES) can function monolithically, or alternatively, in a stackedmode in which, for example, 2-500 MFC units are stacked in order toincrease electrical power output.

The BES (e.g., MFC) can be operated within a mild temperature range ofabout 20 to 50° C. and normal to elevated pressure conditions (i.e.,approximately 1 atm or above). Alternatively, if thermophilic orhyperthermophilic organisms are used, the operating temperature of theBES can be higher (e.g., at about or greater than 50° C., or 60° C., or70° C., or 80° C., or 90° C., or 100° C.). In other embodiments, the BEScan be operated under cooler conditions of less than 20° C., such as atemperature of about or less than 15° C., or 10° C., or 5° C., or 4° C.,or 2° C., or 0° C.

The microbes reside on the anode. Preferably, the microbes are in theform of a biofilm. A biofilm of microbes can be established using any ofthe methods known in the art. For example, as known in the art, abiofilm of microorganisms can be produced on an anode by initiating acolony of microbes on the anode (i.e., by contact of the anode with themicrobes under suitable thriving conditions) and then growing the colonyuntil a biofilm is established on the anode. Preferably, in order tofavor growth of exoelectrogenic microbes, the initiation and growthstage is conducted on the anode while the anode is in electricalcommunication with the cathode. In this way, electrons being donated tothe anode from exoelectrogenic microorganisms can be conducted to thecathode.

A biofilm can be initiated by contact of the anode with an anolyte(i.e., either a specially prepared anodic medium or the effluent) thathas been inoculated with a sampling of microorganisms, at least aportion of which should be capable of operating by an exoelectrogenicmechanism. Preferably, at some point either at the time of contact, orafter contact of the anode with the microorganisms in the anolyte,forced flow and recirculation conditions (i.e., as provided by a pump)are established for the anolyte. For example, in the case of a porousanode, the anolyte is made to flow and recirculate through the anode. Asignificant portion of microorganisms that do not have a strongpropensity for forming biofilms, even though they may be initiallyassociated with the biofilm, will be driven into the anolyte by the flowforce. Accordingly, the forced flow and recirculation conditions of theanolyte serve to enrich the biofilm with microorganisms that have astrong propensity for forming biofilms.

In turn, microorganisms with a strong propensity for funning biofilmsare more likely to contain pili (nanowires) on their external membranewhich can also be used by the microorganisms for direct electrontransfer to the anode. Therefore, the forced flow and recirculationconditions of the anolyte can also serve to further enrich the biofilmwith exoelectrogenic microorganisms capable of direct electron transfer.

At least one advantage of enriching the biofilm with exoelectrogenicmicroorganisms capable of direct electron transfer is that mediators(e.g., ferric oxides, neutral red, anthraquinone dyes,1,4-napthoquinone, thionine, methyl viologen, methyl blue, humic acid,ABTS, and the like) are less needed or completely not needed forfacilitating electron transfer. A mediator-less system is advantageousin that not only are mediators typically expensive, often toxic, andrequire replenishment, but mediated electron transfer is typically lessefficient than direct (mediator-less) electron transfer.

Preferably, in preparing a biofilm on the anode, the flow rate of theanolyte should be high enough to at least maintain planktonicmicroorganisms floating in the medium such that they can be eliminated.A suitable flow rate can be, for example, at least about 2 or 3 mL/min.In different embodiments, the flow rate can be either substantiallyconstant or fluctuating within a range of, for example, 2-10 mL/min, or3-10 mL/min, or 4-10 mL/min, or 5-10 mL/min, or 6-10 mL/min, or 3-8mL/min, or 3-7 mL/min, or 4-8 mL/min, or 4-7 mL/min. The foregoing flowrates are preferably no more than 10 mL/min and are thus herein referredto as a “low flow rate”.

More preferably, in preparing a biofilm on the anode, the flow rate ishigh enough to render those biofilm-forming microorganisms with aresidual level of planktonic ability (i.e., semi-planktonicmicroorganisms) waterborne (i.e., floatational), and thus, removable, asfurther described below. This higher flow rate is preferably above 10mL/min. In different embodiments, the flow rate can be, for example, atleast about 12 mL/min, or at least about 15 mL/min, or at least about 20mL/min, or at least about 25 mL/min, or at least about 30 mL/min, or atleast about 35 mL/min, or at least about 40 mL/min, or at least about 45mL/min, or at least about 50 mL/min. In different embodiments, thehigher flow rate can be either substantially constant or fluctuatingwithin a range of, for example, 12-60 mL/min, 12-50 mL/min, 12-40mL/min, 12-30 mL/min, 12-20 mL/min, 15-60 mL/min, 15-50 mL/min, 15-40mL/min, 15-30 mL/min, 15-20 mL/min, 20-60 mL/min, 20-50 mL/min, 20-40mL/min, 20-35 mL/min, 20-30 mL/min, 25-60 mL/min, 25-50 mL/min, 25-40mL/min, 25-35 mL/min, 25-30 mL/min, 30-60 mL/min, 30-50 mL/min, 30-40mL/min, 35-60 mL/min, 35-50 mL/min, 35-40 mL/min, 40-60 mL/min, 40-50mL/min, 45-60 mL/min, 45-50 mL/min, or 50-60 mL/min.

To reflect changes in volume and cross-sectional area of the MFC(anode), the anolyte flow rate can alternatively be represented in termsof space velocity (cm/min) or hydraulic retention time (HRT in units ofminutes). To convert flow rates given in units of mL/min into spacevelocity, the flow rates are divided by the cross-sectional area of theMFC. For example, for a MFC having a cross-sectional area of 1.25 cm², aflow rate of 2 mL/min corresponds to a space velocity of approximately1.6 cm/min; a flow rate of 10 mL/min corresponds to a space velocity of8 cm/min, and a flow rate of 30 mL/min corresponds to a space velocityof 24 cm/min. To convert flow rates given in mL/min into HRT values, theflow rate is inserted into the following equation: HRT=(volume ofchamber)/(flow rate in mL/min). For example, for a MFC having a chambervolume of 13.25 mL, a flow rate of 2 mL/min corresponds to a HRT valueof approximately 6.6 min.; a flow rate of 10 mL/min corresponds to a HRTvalue of approximately 1.3 min, and a flow rate of 30 mL/min correspondsto a HRT value of approximately 0.44 min.

In a preferred embodiment for preparing a biofilm on the anode, theanolyte is made to flow at any of the low flow rates or ranges thereofdescribed above on a continuous basis (and either a substantiallyconstant or fluctuating basis) along with periodic, intermittent, oroccasional interruptions by any of the higher flow rates or rangesthereof described above. For example, in one embodiment, a continuouslow flow rate in the range of 2-10 mL/min is periodically interrupted bya higher flow rate. The higher flow rate is preferably any of the higherflow rates described above, and more particularly, a flow rate greaterthan 10 mL/min, and more preferably, a flow rate of or greater than 12or 15 mL/min. In a particularly preferred embodiment, the higher flowrate is at least about 35 mL/min. In another embodiment, a continuouslow flow rate in the range of 3-10 mL is periodically interrupted by ahigher flow rate in the range of 30-40, 30-50, or 40-50 mL/min. Inanother embodiment, a continuous low flow rate in the range of 3-7 mL isperiodically interrupted by a higher flow rate in the range of 30-40,30-50, or 40-50 mL/min.

During the forced flow and recirculation conditions for preparing abiofilm on the anode, planktonic microorganisms (i.e., those having apropensity to float in solution rather than form a biofilm) aresubstantially removed by a suitable process (e.g., by use of a syringeor flushing into the effluent water). Preferably, any chemicals that mayfunction as mediators are also removed. For example, in a preferredembodiment, planktonic microorganisms are removed by the periodicreplacement of all or a portion of the flowing and recirculatinganolyte. Since the majority of planktonic microorganisms and anymediators float in solution, periodic replacement of the anolytefunctions to remove these species.

In different embodiments for preparing a biofilm on the anode, theanolyte may be replaced, either by a set or arbitrary number of times ofequivalent volumes of anolyte, or by a set or arbitrary rate ofreplacement such that a substantial absence of planktonic microorganismsin the anolyte in contact with the anode is realized. Preferably, asubstantial absence of planktonic microorganisms corresponds to at leastabout 80%, more preferably at least about 85%, more preferably at leastabout 90%, and even more preferably at least about 95% reduction inconcentration of planktonic microorganisms in the flowing andrecirculating anolyte. For example, in one embodiment, the anolyte issubstantially replaced by 1-20 equivalent volumes of anolyte, either atset intervals or in an arbitrary manner. In another embodiment, theanolyte is substantially replaced (i.e., at least 90% replaced by volumefor each instance of replacement) at specific intervals, such as everyfew minutes, hours, or days. Alternatively, the anolyte can be replacedwhen an optical transmission parameter (property) of the anolyte reachesa level indicative of the presence of planktonic microorganisms. Forexample, a turbidity analyzer (e.g., by laser scattering) or opticaldensity instrument can be used to measure the relative turbidity orlight transmission of the anolyte. In a preferred embodiment, theanolyte is replaced whenever the recirculating anolyte reaches anoptical density (e.g., OD₆₀₀) threshold, e.g., above 0.05 units.Replacement of the anolyte can then be stopped when the optical densityvalue no longer exceeds the desired threshold level.

In a preferred embodiment for preparing a biofilm on the anode, anolyteis replaced directly after the anolyte is subjected to a high flow ratepulse. In this case, the high flow rate pulse can be conducted eitherduring recirculation of the anolyte, or alternatively, during amomentary interruption of the recirculation condition for a timesufficient for the high flow rate pulse (and optionally, a simultaneousor subsequent removal of the medium used for the high flow rate pulse)to take place. The medium used in the high flow rate pulse can be themedium being recirculated or can be a separate medium (e.g., water,purified water, buffered water, or mineralized water) not in contactwith the recirculated medium. In a particular embodiment, the pressureresulting from the high flow rate pulse is used to force the medium outof an outlet of the anodic chamber so that the medium used in the highrate pulse is immediately ejected from the anodic chamber after the highflow rate pulse.

In one embodiment, the microbes residing on the anode have an innateability to efficiently consume one or more mercaptan compounds or CO.The microbes may, in addition, be capable of consuming compounds ormaterials other than mercaptan compounds and/or CO that may also bepresent in the gas stream. In such a case, the microbes may be used inthe treatment process without first cultivating and enriching themicrobes on mercaptans or CO.

In another embodiment, the initial microbe population may not include asufficient number of microbes that possess an ability to effectively orefficiently consume mercaptan compounds or CO. In such a case, themicrobes are preferably cultivated on one or more mercaptan compoundsand/or CO as a nutrient source in order to select and enrich thosemicrobes that possess a tendency or ability to consume mercaptancompounds or CO. For the case of a mercaptan-laden gas stream, themercaptan compounds selected as a nutrient source are preferably thesame (or chemically similar) as the mercaptan compounds to be removedfrom the gas stream. By cultivating microbes on mercaptan compounds orCO, those microbes having a propensity or inclination to catabolizemercaptan compounds or CO undergo a growth stage while microbes not soinclined become weakened by malnutrition and are eventually eliminatedfrom the consortium. In this way, a consortium of microbes enriched inmercaptan-consuming or CO-consuming microbes is produced.

The above microbial growth and enrichment stage is preferably achievedby contacting a microbe-laden anode with a solution or gas streamcontaining one or more mercaptan compounds or CO. The mercaptancompounds or CO can be administered as a sole nutrient source, or alongwith other nutritive compounds. Alternatively, the microbes may beinitially fed a diet based solely or predominantly on one or moretraditional nutritive compounds and thereafter fed a diet more highlyconcentrated in mercaptan compounds or CO. The transition from a dietbased predominantly on non-mercaptan or non-CO compounds to one basedpredominantly on mercaptan compounds or CO can be sudden, incremental,or continuously gradual. The non-mercaptan or non-CO nutritive compoundsare any compounds or materials that can be oxidatively degraded byexoelectrogenic microorganisms such that electrons and protons resultfrom the degradation. The non-mercaptan or non-CO nutritive compoundscan broadly include, for example, waste products (e.g., from sewagestreams, industrial pollutants and byproducts, and foodstuffs),synthetic and natural plastics and polymers, and biological materials.Typically, the non-mercaptan or non-CO nutritive compounds arebiodegradable. The non-mercaptan or non-CO nutritive compounds can beselected from, for example, carboxylic acid-containing compounds ormaterials (e.g., acetic acid, propanoic acid, butyric acid),carbohydrate compounds or materials (e.g., monosaccharides,disaccharides, oligosaccharides, and polysaccharides), lipid-containingsubstances (e.g., fats, mono-, di-, or triglycerides, oils, fatty acids,lipoproteins, or liposaccharides), amino acid-containing substances(e.g., amino acids, dipeptides, tripeptides, oligopeptides, orproteins), or a combination thereof. The growth medium can also containone or more inorganic compounds or materials, such as minerals andvitamins, e.g., alkali and alkaline halide salts (e.g., KCl, MgCl₂, andthe like), phosphates, ammonium salts, and the like.

The microbial growth stage is preferably continued until an electricalcurrent output of the BES or MFC becomes level at a fixed resistancebetween the anode and cathode, after which time the nutritive compoundscan be stopped for a suitable period of time, or periodicallyadministered, in order to maintain a desired current or voltage level.For example, the growth stage may be considered complete when theelectrical output of the BES or MFC stabilizes to a voltage between0.3-0.4 V at a 500 ohm load.

The microorganisms can also be subjected to a starvation stage. Astarvation stage can serve to enrich the microorganisms with a higherproportion of exoelectrogenic microorganisms by weakeningnon-exoelectrogenic organisms and encouraging their elimination.Preferably, the starvation stage is conducted after formation of abiofilm and more preferably after a growth stage, as described above.

The starvation stage is preferably conducted by lowering theadministered amount of nutritive compound (which can include mercaptancompounds or CO, or non-mercaptan or non-CO compounds) to below theamount required for the microorganisms to produce the maximum achievablecurrent under the conditions (e.g., resistance, and other factors)provided by the BES. The amount of nutrient required to produce themaximum achievable current under conditions provided by the BES ishereinafter referred to as the “nutrient threshold value”. Preferably,the administered amount of nutritive source during the starvation stageis no more than 50% of the amount required to attain the nutrientthreshold value. More preferably, the administered amount of nutrientduring the starvation stage is no more than 25%, or no more than 10%, orno more than 1%, of the amount required to attain the nutrient thresholdvalue. Enrichment of the biofilm with exoelectrogenic microorganisms istypically evidenced by maintenance of the voltage output of the BESduring the starvation stage. Preferably, after the initial indication ofa voltage decline, the starvation stage is ended by administration of anamount of nutrient sufficient to at least maintain the voltage of theBES. However, the starvation stage can be ended before a voltage declineis observed, i.e., at a point in time for which it is known that acertain level of enrichment has occurred.

The microorganisms (either in biofilm or planktonic form) can also besubjected to a decreased electrical resistance stage. Lowering theresistance (i.e., load) across the anode and cathode increases thecurrent flow between the two electrodes, and this in turn encourages thegrowth of exoelectrogenic organisms (i.e., further enrichment of themicroorganisms with exoelectrogenic organisms). Preferably, theelectrical resistance is lowered after any of the stages described abovefor producing a biofilm, and more preferably after the starvation stagedescribed above (and more preferably, with reinitiation of the supply ofa nutritive source). The external resistance is typically controlled byuse of a resistor box. The resistor box is preferably one which can beset to any suitable resistance, preferably within the range of 0-5000ohms. The resistance can be reduced by any desirable or suitable amount,either in discrete amounts or gradually over a desired period of time.For example, in different embodiments, the load can be decreased toabout 95%, or 90%, or 85%, or 80%, or 75%, or 70%, or 65%, or 60%, or55%, or 50%, or 45%, or 40%, or 35%, or 30%, or 25%, or 20%, or 15%, or10%, or 5% its original (or full capacity) value.

The electrical power output of the MFC in terms of anode surface area(i.e., the area power density) is preferably at least 2,000 mW/m², andmore preferably, at least 2,500 mW/m², or 3,000 mW/m², or 3,500 mW/rn²,or 4,000 mW/m², or 4,500 mW/m². The area power density can be convertedto volumetric power densities (in units of W/m³) by multiplying the areapower density by the projected surface area (i.e., in m²/m³) and 1/1000.Discussion and examples of projected surface area have been given above.For lower projected surface areas (e.g., 50-500 m²/m³), some values ofvolumetric power density include, for example, 100, 200, 300, 400, 500,600, 700, 800, 900, or 1,000 W/m³, or a range resulting from any two ofthese values. For higher projected surface areas (e.g., 1,000-100,000m²/m³), some values of volumetric power density include, for example,2,000, 5,000, 10,000, 20,000, 50,000, 100,000, 150,000, 200,000,250,000, 300,000, 350,000, or 400,000 W/m³, or a range resulting fromany two of these values. Typically, the volumetric power density isrecited in terms of net anode volume (NAV), which is the void volumefraction (i.e., volume fraction not occupied by liquid) in the anode. Toconvert the volumetric power density to reflect the NAV, the volumetricpower density is divided by the void volume, which has the effect ofincreasing the value of the volumetric power density. Typically, thevoid volume is at or above 25% and up to about 95% (i.e., typically anyvalue within 0.25 to 0.95). Some void volume values include, forexample, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%,85%, 90%, and 95%, or values above or below any of these values, or anyrange resulting from any two of these values.

The electricity generated by the BES can be used to power any desiredprocess. Preferably, the electricity is used to power an aspect of theprocess from which the mercaptan-laden or CO-laden gas stream emanates.For example, the electricity can be used to heat, pressurize, condense,or transport one or more components in an industrial process, such asany of the energy-demanding operations commonly employed in an HDSprocess, Claus process, Merox process, steam-methane reforming process,catalytic water-gas shift process, or Fischer-Tropsch process.

The gas stream can also be suitably modified before or during processingby the BES. The modification of the gas stream can, for example, enhanceor promote BES operation as compared to BES operation using anunmodified gas stream. In one embodiment, the gas stream is modified bybeing diluted. Dilution may be advantageous in a situation where the BEShas a threshold tolerance to a mercaptan, CO, or other chemical presentin the gas stream, wherein dilution causes the concentration of themercaptan, CO, or other chemical to be less than the threshold level. Inanother embodiment, the gas stream is processed by an adsorptive processfor removal of one or more chemicals from the gas stream. In anotherembodiment, the gas stream is supplemented with one or more additionalnutritive chemicals. In yet another embodiment, the gas stream isprocessed by a pre-oxidation process (e.g., bleaching, peroxidation,ozonation, hot filament oxygenation). A pre-oxidation process can beparticularly advantageous when mercaptan compounds resistant tooxidative degradation are present, or when the microbes being usedcannot efficiently or effectively consume the mercaptan compoundsdirectly. The oxidation process may also be used as a post-treatmentstep (i.e., after microbial degradation) to ensure complete oxidation ofone or more compounds. Any one or more of the gas pre- or post-treatmentsteps can be partially or completely powered by electricity produced byone or more BESs consuming mercaptan compounds or CO.

In a preferred embodiment, the gas stream is pre-treated by adjustingits humidity level before entry of the gas stream into the BES. Thehumidity level of the gas stream is preferably adjusted to at least 40%.In different embodiments, the humidity level of the gas stream ispreferably adjusted to at least 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%,85%, 90%, 95%, 97%, 98%, or 99%. In a particular embodiment, thehumidity level of the gas stream is adjusted to the saturation point ofthe gas stream under the conditions (e.g., temperature and pressure)being used.

The humidity level of the gas stream can be suitably increased ordecreased by any suitable method. The humidity level of the gas streamcan be suitably decreased by, for example, passing the gas streamthrough a condenser or water-absorption unit (e.g., calcium sulfatehemihydrate, silica gel, or the like). The humidity level of the gasstream can be suitably increased by, for example, passing the gas streamthrough a column of water. In order to increase the water absorptionrate of the gas stream, the gas stream can be suitably mixed or agitatedin the column of water to promote interaction of the gas stream with thewater. The column of water can also be adjusted in height (of the waterlevel) and in buoyancy (e.g., by inclusion of a viscosifier) to affectthe retention time of the gas stream in the water. The temperature ofthe water can also be suitably adjusted, preferably to within a range of30-50° C., and more preferably about 40° C., particularly when watersaturation is desired. However, lower temperature water (e.g., less than30° C.) or higher temperature water (e.g., higher than 50° C.) can alsobe used. In another embodiment, the humidity of the gas stream isincreased by mixing the gas stream with water vapor or steam.

Alternatively, the BES or MFC can be operated in a manner such that acathode therein, instead of producing water by absorption of oxygen,produces hydrogen gas. When hydrogen is produced, such a device can bereferred to herein as a microbial electrolysis cell (MEC). In order torender the BES capable of producing hydrogen gas, the BES or MFC ispreferably modified in one or more ways, as follows: i) the cathode ispreferably constructed of a hydrogen-producing material; ii) the cathodeis preferably substantially deoxygenated; and iii) the cell potential ofthe BES or MFC is preferably adjusted, by application of an externalvoltage, such that hydrogen can be produced at the cathode. Thehydrogen-producing material of the cathode can be any material capableof producing hydrogen from protons, including, for example,hydrogen-producing metals, such as typically platinum (Pt), palladium(Pd), nickel (Ni), iron (Fe), copper (Cu), or an alloy thereof. Otherpossible hydrogen-producing materials include hydrogen-producing enzymesor microbes. An anaerobic environment at the cathode may be providedwithout special measures if the conditions in which the BES or MFC isemployed are naturally anaerobic. However, if anaerobic conditions arenot naturally present, deoxygenation of the cathode can be convenientlyachieved by, for example, sparging of the catholyte with nitrogen (orother inert gas) and/or sealing of the cathodic chamber so as to prevententry of air. The cell potential (ΔV) can be adjusted such that the BESor MFC anode potential (typically around −0.3 V) is externallysupplemented such that the potential difference between the anode andthe cathode is at least about 0.41 V. For example, the anode can beexternally assisted with a voltage of about 0.11 to 0.25 V or higher forthis purpose, depending on the magnitude of the cathode overpotential.For hydrogen production, the anolyte and catholyte need not be separatedby a cation-selective permeable membrane.

The hydrogen gas, in turn, can be used for any purpose, e.g., as acommodity chemical, a reactant or processing chemical in the process towhich the BES is applied (or another process), or as a fuel source,particularly for powering one or more processes associated with the gasstream being processed. As a reactant in the process, the hydrogen gascan be used for hydrogenation (e.g., in an HDS or Fischer-Tropschprocess), or as part of a reductive process, or as a feedstock foranother MFC. As a fuel source, the hydrogen gas can be combusted with areactive (i.e., oxidizing) gas, or reacted with a gas to produce a fuelwhich is then combusted, e.g., the physical-, enzymatic-, ormicrobial-mediated production of methane (natural gas) from syngas(i.e., CO+H₂).

In a particular embodiment, the produced hydrogen is used as anelectricity-generating fuel to power one or more processes, particularlythose process steps from which the gas stream emanates. For example, theproduced hydrogen can be directed into a fuel cell capable of reactinghydrogen with a suitable oxidant (typically oxygen) to produce waterwhile generating electricity. The fuel cell can be any suitable fuelcell known in the art capable of reacting hydrogen gas with an oxidantto make electricity. The fuel cell can be, for example, ahydrogen-oxygen proton exchange membrane (PEM) fuel cell, an alkalinefuel cell, metal hydride fuel cell, molten carbonate fuel cell, or solidoxide fuel cell. The fuel cell that uses the produced hydrogen as a fuelcan also be a microbial fuel cell capable of using hydrogen as a fuel,i.e., which contains microbes capable of using hydrogen as a nutrientsource.

The BES can also be operated in a manner such that one or moreelectrochemically reducible compounds or materials is reduced at thecathode. Preferably, a cathode of the BES is operated in the substantialabsence of oxygen for this purpose. The reductive process is preferablyused for the breakdown or elimination of one or more chemical speciesthat are deleterious to the BES or that are environmentally malignant.The environmentally malignant species can be, for example, a degradationproduct, a pollutant, waste product, or toxin. Some examples ofreductive processes include nitrate reduction, uranium reduction andperchlorate reduction (Rabaey, K. et al. The ISME Journal 1, 9-18(2007)).

In one embodiment, the reducible species is a compound or materialcontaining a nitrogen oxide (N—O) bond. Such a compound is commonly anitrate-containing species (i.e., “a nitrate” or “nitrate compound”).The nitrate compounds can include inorganic nitrate species (e.g.,NaNO₃, KNO₃, NH₄NO₃, Mg(NO₃)₂, AgNO₃, HNO₃, and so on) as well asorganonitrate species, such as tetramethylammonium nitrate. Other typesof nitrogen oxide compounds that can be reduced include the nitrites,organonitro compounds, dinitrogen tetroxide, nitrosyl (nitroso)compounds, nitric oxide (NO), and nitrosonium species.

In another embodiment, the reducible species is a compound or materialcontaining a halogen oxide bond. A common class of such compounds arethe chlorine oxide class of compounds. A common subclass of chlorineoxide compounds are the perchlorates. The perchlorates include inorganicperchlorate species (e.g., LiClO₄, NaClO₄, KClO₄, NH₄ClO₄, Mg(ClO₄)₂,AgClO₄, HClO₄, and so on) as well as organoperchlorate species, such astetramethylammonium perchlorate. Other subclasses of chlorine oxidecompounds include the chlorates, chlorites, hypochlorites, and theiracids. Other classes of halogen oxide compounds include the bromineoxide and iodine oxide classes of compounds. Some subclasses of bromineoxide compounds include the perbromates, bromates, bromites,hypobromites, and their acids. Some subclasses of iodine oxide compoundsinclude the periodates, iodates, iodites, hypoiodites, and their acids.

In another embodiment, the reducible species is a compound (e.g., salt)or material containing one or more reducible metal species. A reduciblemetal species typically contains a metal atom having a positiveoxidation state. The reductive method is particularly effective inreducing heavy metals, which are often harmful to the environment and inneed of removal. Some examples of reducible metal species include Cr(VI)as found in chromates and dichromates, Mn(VII) as found inpermanganates, Fe(III), Ni(III), Cu(II), Cu(I), Pd(II), Ag(I), Cd(II),Au(III), Au(I), Hg(I), Pb(II), and U(VI), which can be converted to therelatively insoluble U(III) species. The more reducible heavy metals canbe reduced to elemental form, which can allow for their more facileremoval.

In yet another embodiment, the reducible species is a peroxide. Theperoxide can be, for example, inorganic (e.g., hydrogen peroxide), or anorganoperoxide, such as carbamide peroxide, dibenzoyl peroxide, andcumene hydroperoxide.

In still another embodiment, the reducible species is a reduciblesulfurous substance. The sulfurous substance can be, for example, sulfurdioxide, sulfur trioxide, sulfuric acid, a sulfate, a sulfite, abisulfite, a persulfate (e.g., a peroxodisulfate), or a disulfide. Thesulfurous substance may result from the oxidative degradation ofmercaptans by the MFC, or from the process being treated. In the casethat the sulfurous substance is the result of oxidative degradation ofmercaptans by the BES, the sulfurous substance is preferably not reducedback to thiols, as this would render the process inefficient. However,particularly if the sulfurous substance is gaseous (e.g., SO₂) andoriginates from the process being treated, it may be advantageous forthe cathode to reduce the sulfurous substance to a thiol, and direct thethiol to the anode where it can be oxidized to a non-gaseous product(e.g., a sulfite or sulfate).

The microorganisms (i.e., organisms) that are used in the MFC can be anysuitable microorganisms. The microorganism can be, for example,eukaryotic or procaryotic, and either unicellular or multicellular. Anexample of a suitable unicellular eukaryotic microorganism is yeast.Other examples of unicellular eukaryotic microorganisms include theprotists or protozoans, such as amoeba and paramecia. An example ofmulticellular eukaryotic microorganisms includes the euglena. Thosealgae capable of uptake of organic carbon (e.g., eukaryotic orprocaryotic mixotrophic forms) are also contemplated herein. Procaryoticorganisms are predominantly unicellular, and are divided into twodomains: the bacteria and the archaea. The procaryotic organisms canalso be broadly divided into four main groups according to their shape:the cocci, the bacilli, spirilla, and vibrio. The archaea include theextremophiles (e.g., as found in hot springs and lakes), and thenon-extremophiles, as found in soil, the oceans, and marshland. Thearchaea also include the methanogens.

In one embodiment, the microorganisms considered herein are bacteria.Some examples of phyla of bacteria considered herein are theAcidobacteria, Actinobacteria, Aquificae, Bacteroidetes, Chlorobi,Chlamydiae/Verrucomicrobia, Chloroflexi, Chrysiogenetes, Cyanobacteria,Deferribacteres, Deinococcus-Thermus, Dictyoglomi, Fibrobacteres,Firmicutes, Fusobacteria, Gemmatimonadetes, Nitrospirae, Planctomycetes,Proteobacteria (α, β, γ, δ varieties), Spirochaetes, Synergistetes,Tenericutes, Thermodesulfobacteria, Thermotogae, or any combinationthereof. Some particularly relevant families of bacteria beingconsidered herein include Acidaminococcaceae, Acidobacteriaceae,Aeromonadaceae, Alteromonadaceae, Clostridiaceae, Comamonadaceae,Desulfobulbaceae, Desulfuromonadaceae, Enterobacteriaceae,Geobacteraceae, Pasturellaceae, Pelobacteraceae, Pseudomonadaceae,Rhodocyclaceae, and Shewanellaceae. Any combination of bacteriacontaining at least one of the above families of bacteria are alsocontemplated herein.

In a particular embodiment, the microbes include bacteria from thephylum Firmicutes. Some particular classes of Firmicutes bacteria beingconsidered herein are Bacilli, Clostridia, and Mollicutes. A particularorder of Clostridia being considered herein is Clostridiales. Someparticular families of Clostridiales being considered herein areAcidaminococcaceae, Clostridaceae, and Veillonellaceae. Some particulargenera of Acidaminococcaea or Veillonellaceae being considered hereinare Acetonema, Acidaminococcus, Allisonella, Anaeroarcus, Anaeroglobus,Anaeromusa, Anaerosinus, Anaerovibrio, Centipeda, Dendrosporobacter,Dialister, Megamonas, Megasphaera, Mitsuokella, Pectinatus, Pelosinus,Phascolarctobacterium, Propionispira, Propionispora, Quinella,Schwartzia, Selenomonas, Sporomusa, Sporotalea, Succiniclasticum,Succinispira, Thermosinus, Veillonella, and Zymophilus. Some particulargenera of Clostridaceae being considered herein are Acetanaerobacterium,Acetivibrio, Acidaminobacter, Alkaliphilus, Anaerobacter, Anaerotruncus,Anoxynatronum, Bryantella, Caldanaerocella, Caloramator,Caioranaerobacter, Caminicella, Candidatus Arthromitus, Clostridium,Coprobacillus, Dorea, Ethanologenbacterium, Faecalibacterium, Garciella,Guggenheimella, Hespellia, Linmingia, Natronincola, Oxobacter,Parasporobacterium, Sarcina, Soehngenia, Sporobacter, Subdoligranulum,Tepidibacter, Tepidimicrobium, Thermobrachium, Thermohalobacter, andTindallia.

In another particular embodiment, the microbes include one or moreclasses of bacteria from the phlyum Proteobacteria.

A particular class of Proteobacteria being considered herein is AlphaProteobacteria. Some particular orders of Alpha Proteobacteria beingconsidered herein are Caulobacterales (e.g., the familyCaulobacteraceae, or Caulobacter sp.), Kordiimonadales, Parvularculales,Rhizobiales (e.g., the family Rhizobiaceae, or Rhizobium sp.),Rhodobacterales, Rhodospirillales (e.g., the family Acetobacteraceae, orAcetobacter sp.), Rickettsiales (e.g., the family Rickettsiaceae, orRickettsia sp.), and Sphingomonadales (e.g., the familySphingomonadaceae, or Sphingomonas sp.), wherein “sp.” or “spp.” as usedherein both indicate one or more species of the indicated genus.

Another particular class of Proteobacteria being considered herein isBeta Proteobacteria. Some particular orders of Beta Proteobacteria beingconsidered herein are Burkholderiales, Hydrogenophilales,Methylophilales, Neisseriales (e.g., the family Neisseriaceae, orNeisseria sp.), Nitrosomonadales, Rhodocyclales, and Procabacteriales. Aparticular family of Burkholderiales being considered herein isComamonadaceae. Some particular genera of Comamonadaceae beingconsidered herein are Acidovorax, Aquabacterium, Brachymonas, Comamonas,Curvibacter, Delftia, Hydrogenophaga, Ideonella, Leptothrix, Malikia,Pelomonas, Polaromonas, Rhodoferax, Roseateles, Sphaerotilus,Tepidimonas, Thiomonas, and Variovorax. A particular family ofRhodocyclales being considered herein is Rhodocyclaceae. A particulargenus of Rhodocyclaceae being considered herein is Azospira.

Another particular class of Proteobacteria being considered herein isGamma Proteobacteria. Some particular orders of Gamma Proteobacteriabeing considered herein are Acidithiobacillales, Aeromonadales,Alteromonadales, Cardiobacteriales, Chromatiales (purple sulfurbacteria), Enterobacteriales (e.g., the family Enterobacteriaceae, suchas the genera Escherichia or Salmonella), Legionellales (e.g., thefamily Legionellaceae, or Legionella sp.), Methylococcales,Oceanospirillales, Pasteurellales (e.g., the family Pasteurellaceae, orHaemophilus sp.), Pseudomonadales, Thiotrichales (e.g., Thiomargarita),Vibrionales (e.g., the family Vibrionaceae, or Vibrio sp.),Xanthomonadales (e.g., the family Xanthomonadaceae, or Xanthomonas sp.).A particular family of Aeromonadales being considered herein isPseudomonadaceae. A particular genus of Pseudomonadaceae beingconsidered herein is Pseudomonas (e.g., P. aeruginosa). Some particularfamilies of Alteromonadales being considered herein are Shewanellaceaeand Pseudoalteromonas. A particular genus of Shewanellaceae beingconsidered herein is Shewanella (e.g., S. putrefaciens).

Another particular class of Proteobacteria being considered herein isDelta Proteobacteria. Some particular orders of Delta Proteobacteriabeing considered herein are Aeromonadales, Bdellovibrionales (e.g., thefamily Bdellovibrionaceae, or Bdellovibrio sp.), Desulfobacterales,Desulfovibrionales, Desulfurellales, Desulfarcales, Desulfuromonadales,Myxococcales (Myxobacteria), and Syntrophobacterales. A particularfamily of Aeromonadales being considered herein is Aeromonadaceae. Aparticular genus of Aeromonadaceae being considered herein is Aeromonas.Some particular families of Desulfuromonadales being considered hereinare Desulfuromonadaceae, Pelobacteraceae, and Geobacteraceae. Aparticular genus of Desulfuromonadaceae being considered herein isDesulfuromonas. A particular genus of Geobacteraceae being consideredherein is Geobacter (e.g., Geobacter sulfurreducens and Geobactermetallireducens). A particular family of Desulfobacterales beingconsidered herein is Desulfobulbaceae. A particular genus ofDesulfobulbaceae being considered herein is Desulfobulbus.

Another particular class of Proteobacteria being considered herein isEpsilon Proteobacteria. Some particular orders of Epsilon Proteobacteriabeing considered herein are Campylobacterales (e.g., the familyHelicobacteraceae, or Helicobacter sp.) and Nautiliales.

In another particular embodiment, the microbes include one or morebacteria from the phlyum Acidobacteria. A particular order ofAcidobacteria being considered herein is Acidobacteriales. A particularfamily of Acidobacteriales being considered herein is Acidobacteriaceae.Some particular genera of Acidobacteriaceae being considered herein areAcidobacterium, Geothrix, Holophaga, and Chloracidobacterium.

In another particular embodiment, the microbes include one or morethermophilic bacteria from the order Thermotogales. Some particulargenera of Thermotogales being considered herein are Thermotoga,Caldotoga, Fervidobacterium, Geotoga, Marinitoga, Petrotoga,Thermopallium, and Thermosipho. A related family of thermophilicbacteria being considered herein is Thermoanaerobiaceae. Some particulargenera of Thermoanaerobiaceae being considered herein areThermoanaerobacter and Thermoanaerobacterium. Some particular species ofThermoanaerobacter being considered herein are Thermoanaerobacterthermohydrosulfuricus, Thermoanaerobacter subterraneus,Thermoanaerobacter brockii, Thermoanaerobacter yonseiensis, andThermoanaerobacter tengcongensis.

The above microbes can all be considered for consumption of mercaptancompounds or CO. However, in the particular case of CO processing, thereare some types of microbes that are more suited for effecting awater-gas shift reaction than the microbes considered thus far. Somemicrobes particularly considered for utilizing CO as a nutrient sourceinclude, for example, Rubrivivax gelatinosus, Butyribacteriummethylotrophicum, Clostridial bacteria, and Rhodospirillus rubrum.

The microbes used in the BES can be selective or non-selective withrespect to oxidative degradation of mercaptan compounds. For example, aconsortium or species of microbes may be used which is generallynon-selective in its ability to oxidatively degrade mercaptan compounds,i.e., the microbes can oxidatively degrade a wide number of differentmercaptan compounds. A consortium or species of microbes may also besomewhat selective in processing mercaptan compounds in that themicrobes may oxidatively degrade one or more types of mercaptancompounds more efficiently or effectively than one or more other typesof mercaptan compounds. Finally, a consortium or species of microbes maybe highly selective in processing one or more specific mercaptancompounds while being essentially inefficient or ineffective inprocessing one or more other mercaptan compounds.

In one embodiment, a population of microbes incorporated into the BES orMFC is relatively homogeneous by having a predominant proportion of themicrobe population (typically at least 90%, 95%, 97%, 98%, or 99%)within a particular class, order, family, genus, or species ofmicroorganism. In another embodiment, a population of microbesincorporated into the BES or MFC is relatively heterogeneous (i.e., aconsortium of microbes). A relatively homogeneous or heterogeneoussample of microbes can be obtained by any method known in the art,including as a purified culture (i.e., as prepared by cell culturingmethods) or from a non-cultured source. Some examples of non-culturedsources from which a population of microbes can be obtained for the BESor MFC include, for example, a waste stream (e.g., municipal orindustrial waste streams), top soil, hot spring, estuary, deep sea vent,underground environment, or a contaminated environment (e.g., amercaptan-contaminated environment).

In one embodiment, a single BES or MFC is capable of oxidativelydegrading a wide number of mercaptan compounds and/or CO, therebyenabling the BES or MFC to remove the majority (or all) of the differenttypes of mercaptan compounds and possibly CO that may be present in thegas stream. Alternatively, a multiplicity (i.e., system) of BESs or MFCscan all have the same ability to degrade the same types of mercaptancompounds. In another embodiment, a multiplicity of BESs or MFCs (i.e.,two or more), each containing microbes that are specialized fordegrading specific mercaptan compounds, are used in the process toremove the majority (or all) of the different types of mercaptancompounds that may be present in the gas stream.

FIG. 1 depicts a particular preferred embodiment of an MFC design,hereinafter referred to as a “horizontal gas-phase MFC configuration”.This design facilitates liquid flow (i.e., proton-conducting liquid)from the anode through the membrane to the cathode by the action ofgravity. This configuration keeps the cathode moist by diffusion ofliquid from the anode, and hence, permits efficient proton transfer tothe cathode from the anode. An advantage of this configuration is thatthe cathode does not require separate wetting. However, since the liquidwetting the cathode comes from the anode, this configuration preferablyincorporates a cation-exchange membrane between the anode and cathode inorder to prevent anionic species from the anode solution to contact thecathode.

FIG. 2 depicts another particular preferred embodiment of an MFC design,hereinafter referred to as a “vertical gas-phase MFC configuration”.This design is particularly suitable for high throughput processing of agas stream having a low H₂S content. Since in this design gravity is notbeing used to facilitate liquid flow from the anode to the cathode, thecathode typically requires separate wetting. Therefore, acation-exchange membrane is not necessary in this configuration.Instead, a cation-permeable membrane may be used. The cation-permeablemembrane can be, for example, filter paper, preferably having a poresize of 0.2 microns or less.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

1. A method for producing electrical energy or hydrogen gas from a gasstream containing one or more gaseous compounds that are oxidativelydegradable by microbes, the method comprising contacting the gas stream,as a gas-continuous stream, with an anode of a bioelectrochemicaldevice, said anode containing said microbes which oxidatively degradeone or more of said gaseous compounds while producing electrical energyor hydrogen gas by said oxidative degradation, wherein: (i) said anodeis sufficiently porous such that gas is permitted to flow therethrough,(ii) said anode contains on its surface and/or interior portions aproton-conducting liquid film that is maintained by an anode wettingprocess to maintain the liquid film, and (iii) said anode is inelectrical communication with a cathode of the bioelectrochemicaldevice.
 2. The method of claim 1, wherein said microbes are in the formof a biofilm on said anode.
 3. The method of claim 1, wherein said oneor more gaseous compounds comprise a mercaptan compound and/or carbonmonoxide.
 4. The method of claim 1, wherein said anode comprises a formof elemental carbon.
 5. The method of claim 1, wherein said anode is athree-dimensional electrode.
 6. The method of claim 1, wherein saidanode has been previously rendered hydrophilic by a suitable surfacetreatment process.
 7. The method of claim 6, wherein the surfacetreatment process is a plasma treatment process.
 8. The method of claim1, wherein the anode possesses a porosity value of at least 50%.
 9. Themethod of claim 1, wherein the anode possesses a specific surface areaof at least 5,000 m²/m³.
 10. (canceled)
 11. The method of claim 1,wherein said proton-conducting liquid film has a thickness in the rangeof 1 micron and up to 10 microns.
 12. The method of claim 1, furthercomprising controlling the humidity level of the gas stream such thatthe humidity of the gas stream is at least 40% before entry of the gasstream into the bioelectrochemical device.
 13. The method of claim 12,wherein the humidity level of the gas stream is controlled by passingthe gas stream through a column of water.
 14. The method of claim 1,wherein the gas stream emanates from a petroleum refining operation. 15.The method of claim 1, wherein electricity or hydrogen produced from thebioelectrochemical device is used to power one or more mechanismsinvolved in an operation that produces the gas stream.
 16. The method ofclaim 1, wherein the bioelectrochemical device is operated such thathydrogen gas is produced at the cathode by operating thebioelectrochemical device under the conditions that the cathode isconstructed of a hydrogen-producing material and is deoxygenated, andthe cell potential of the bioelectrochemical device is adjusted byapplication of an external voltage such that hydrogen is produced at thecathode.
 17. The method of claim 1, wherein the bioelectrochemicaldevice is operated such that the cathode electrochemically reduces oneor more electrochemically reducible species other than hydrogen ions.18. The method of claim 1, wherein a cation-permeable material is indirect contact with said anode and cathode, and separates said anode andcathode. 19.-37. (canceled)
 38. The method of claim 1, wherein said gasstream contains carbon monoxide.
 39. The method of claim 38, whereinsaid gas stream emanates from a biomass gasification operation.
 40. Themethod of claim 38, wherein the microbes on the anode are capable ofeffecting a water-gas shift reaction.
 41. The method of claim 40,wherein the microbes are selected from the group consisting ofRubrivivax gelatinosus, Butyribacterium methylotrophicum, Clostridia,and Rhodospirillus rubrum.
 42. The method of claim 1, wherein said gasstream contains one or more mercaptans.
 43. The method of claim 42,wherein said gas stream is substantially absent of carbon monoxide.